the electric plug cover.........three small bamboo canes inserted into the holes and hey presto instant mini obelisk for small house plants!
polystyrene; great for standing overwintering on in a cold greenhouse and keeps their "feet" warm. Broken into pieces (or sometimes items are packed in chippings) and put into the base of large pots that are only going to house "annuals" means you use less compost and makes the pots lighter to carry.
The endless plastic bags....when germinating seeds or propagating from cuttings; place a stick in the pot, cover with plastic bag, put an elastic band round the bag and pot and you have an instant mini propagator.
And any cardboard can either be torn up and put on the compost heap (it will take some time to break down though) or many councils will take them if you have recycling dustbin collections.